Experiment 3: Reconsolidation of Fix-C and Esc-C memory

Administration of MK-801 before the CPP test (pre memory retrieval) prevented
a reduction in place preference for Fix-C [F(1,48)=8.904; p=0.004; group effect] but it had
no effect on Esc-C (Fig 2.3B,C). For Fix-C, there was a significant time effect
[F(2,48)=48.437; p<0.001] and a significant group x time interaction [F(2,48)=3.246;
p=0.048]. For Esc-C, there was a significant time effect [F(2,33)=85.508; p<0.001] but no
significant group x time interaction (p>0.05). The results suggest that the first exposure to
the CPP context may elicit extinction in the Fix-C but not Esc-C group and MK-801
inhibited the extinction in the Fix-C group.
In the next experiment MK-801 was administered 6 min following the first CPP
test (post memory retrieval). A dose of 0.1mg/kg MK-801 attenuated subsequent
expression of Fix-C. Two-way ANOVA showed a significant time effect [F(2,39)=43.151;
p<0.001] but no significant group effect nor group x time interaction (p>0.05). Post hoc
analysis showed that on day 12, MK-801-treated animals showed reduced CPP compared
to saline-treated mice (p=0.012), suggesting that MK-801 disrupted memory
reconsolidation. With respect to Esc-C, two-way ANOVA comparing the dose effect of
MK-801 on disruption of reconsolidation showed a significant group (MK-801 dose)
effect [F(2,66)=5.747; p=0.005], a significant time effect [F(2,66)=83.735; p<0.001] but no
significant group x time interaction. Results suggest that while a low dose (0.1mg/kg)
MK-801 was sufficient to disrupt Fix-C memory reconsolidation, a higher dose of MK-
801 (0.3mg/kg) was required for disruption of Esc-C memory reconsolidation (Fig. 2.3E;
day 12).
In all subsequent reconsolidation experiments drugs were administered following
a 6 min CPP test (post memory retrieval). For both Fix-C and Esc-C, the 6 min exposure
to the context was sufficient to express the acquisition of CPP (Pre-CPP vs Retrieval;
p<0.001; Fig 2.4). Ifenprodil (group effect) significantly disrupted reconsolidation of
both Fix-C and Esc-C memory [for Fix-C:F(1,48)=4.123; p=0.048; for Esc-
C:F(1,39)=11.832; p=0.001; Fig 4B, C]. There was a significant time effect [for Fix-
C:F(2,48)=31.171; p<0.001; for Esc-C:F(2,39)=25.697; p<0.001] but no significant group x
time interaction. Administration of ifenprodil either in the home cage in the absence of
memory retrieval or 6h after memory retrieval had no effect on Esc-C CPP (Fig 4D),
suggesting that ifenprodil disrupted memory reconsolidation. To further validate the
involvement of NR2B in Fix-C and Esc-C memory reconsolidation, the effects of
traxoprodil was investigated. Traxoprodil is another NR2B receptor antagonist which has
previously been tested in clinical trials for the treatment of depression (Preskorn et al.,
2008), dyskinesia and Parkinsonism (Nutt et al., 2008). Administration of traxoprodil
following memory retrieval significantly reduced subsequent expression of CPP in both
Fix-C and Esc-C (Fig 2.4E, F). Two-way ANOVA showed a significant group effect for
Esc-C [F(1,42)=9.466; p=0.004] but not Fix-C [F(1,42)=2.555; p=0.117]. There was a
significant time effect for both Esc-C [F(2,42)=75.191; p<0.001] and Fix-C [F(2,42)=35.280;
p<0.001] but no significant group x time interaction in either case. Post hoc analysis
showed that traxoprodil-treated mice displayed significantly lower CPP scores than
vehicle-treated mice for both Esc-C (p<0.001) and Fix-C (p=0.005) on day 12.
We then sought to isolate the contribution of different signaling molecules
downstream of the NMDAR in cocaine-memory reconsolidation. Post-retrieval
administration of the nNOS inhibitor 7-NI reduced the magnitude of subsequent CPP that
was acquired by Fix-C but not Esc-C schedule (Fig. 2.5B, C), suggesting disruption of
Fix-C [F(1,45)=6.558; p=0.014] but not Esc-C [F(1,48)=0.0166; p=0.898] memory
reconsolidation. There was a significant time effect for Fix-C [F(2,45)=42.089; p<0.001]
and Esc-C [F(1,48)=29.907; p<0.001] but no significant group x time interaction. The
disruption of Fix-C memory reconsolidation by 7-NI is consistent with our previous
studies (Itzhak & Anderson, 2007).
We then investigated the involvement of ERK in reconsolidation of Fix-C and
Esc-C memory. The ERK kinase (MEK) inhibitor SL327 disrupted Esc-C but not Fix-C
memory reconsolidation (Fig 2.5D, E). Two-way ANOVA showed no significant group
effect for Esc-C [F(1,36)=3.904; p=0.056] nor Fix-C [F(1,51)=0.873; p=0.354]. However,
there was a significant time effect [for Esc-C:F(2,36)=49.884; p<0.001; for Fix-
C:F(2,51)=63.774; p<0.001] and a significant group x time interaction for Esc-C
[F(2,36)=3.727; p=0.034] but not for Fix-C [F(2,51)=0.344; p=0.71]. Post hoc analysis found
a significant reduction in CPP on day 12 for Esc-C (p=0.014) but not Fix-C (p>0.05).
Taken together, these results suggest that Fix-C memory engages NO signaling while
Esc-C memory may bypass the dependence on NO and engages the NMDAR-dependent
ERK signaling pathway.


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